There's a SOUPS paper on the OLPC security model that might be pertinent
to the current discussions. I'm yet to read it but it apparently
considers the OLPC security architecture from a usability point-of-view
(which makes sense given the forum). Hence, may be relevant to the
"solutions motivated by use cases" thread.
Further, it appears relevant to this current thread. The abstract of the
paper reads:
"We present an integrated security model for a low-cost lap-
top that will be widely deployed throughout the developing
world. Implemented on top of Linux operating system, the
model is designed to restrict the laptop’s software without
restricting the laptop’s user."
PDF available here:
http://cups.cs.cmu.edu/soups/2007/proceedings/p132_krstic.pdf